Don't be alarmed if you see dead fish along Lake Michigan.
These fish are alewives, and they are invasive. Alewives were first found in the lake in 1949, and they now make up about 50% of all the fish biomass.
Living in freshwater is believed to be stressful to the fish, and changes in water temperature and oxygen levels can harm them, so they die.
These seasonal die-offs were more frequent 20-60 years ago, the DNR said, but are rarer now. This year is an exception.
"The die-off is larger than normal this year and something we have not seen in years," said Jay Wesley, Lake Michigan basin coordinator for the Michigan Department of Natural Resources. "We are seeing the die-off extend from Muskegon all the way up to Cross Village and out to the Beaver Island complex."
The DNR believes this is a natural event and not caused by pollution or disease.
If you suspect a fish kill is due to non-natural causes, call the nearest DNR office or Michigan's Pollution Emergency Alert System at 800-292-4706.